Methods for processing a wide band gap semiconductor wafer, methods for forming a plurality of thin wide band gap semiconductor wafers, and wide band gap semiconductor wafers

ABSTRACT

A method for processing a wide band gap semiconductor wafer is proposed. The method includes depositing a non-monocrystalline support layer at a back side of a wide band gap semiconductor wafer, depositing an epitaxial layer at a front side of the wide band gap semiconductor wafer, and splitting the wide band gap semiconductor wafer along a splitting region to obtain a device wafer including at least a part of the epitaxial layer, and a remaining wafer including the non-monocrystalline support layer.

TECHNICAL FIELD

Examples relate to methods for processing a wide band gap semiconductorwafer, to methods for forming a plurality of thin wide band gapsemiconductor wafers, and to wide band gap semiconductor wafers.

BACKGROUND

Thin semiconductor wafers, for example of a thickness of less than 250μm, may show a wafer bow or warp. Consequently, it might not be possibleto process thin semiconductor wafers fully automatically as processequipment might not be able to handle such thin semiconductor wafers.Semiconductor wafers may be thinned by wafer splitting, for example toenable reuse.

SUMMARY

An example relates to a method for processing a wide band gapsemiconductor wafer. A non-monocrystalline support layer is deposited ata back side of a wide band gap semiconductor wafer. Further, anepitaxial layer is deposited at a front side of the wide band gapsemiconductor wafer. The method comprises splitting the wide band gapsemiconductor wafer along a splitting region to obtain a device wafercomprising at least a part of the epitaxial layer, and to obtain aremaining wafer comprising the non-monocrystalline support layer.

An example relates to a method for forming a plurality of thin wide bandgap semiconductor wafers. The method comprises depositing a firstnon-monocrystalline support layer on a wide band gap semiconductorboule. The wide band gap semiconductor boule is separated along a firstseparating region to obtain a first thin wide band gap semiconductorwafer comprising the non-monocrystalline support layer and a thin wideband gap semiconductor layer, and to obtain a first remaining wide bandgap semiconductor boule. For example, a thickness of the wide band gapsemiconductor boule is at least 2 times a thickness of the thin wideband gap semiconductor layer. A further non-monocrystalline supportlayer is deposited on the first remaining wide band gap semiconductorboule. The first remaining wide band gap semiconductor boule isseparated along a further separating region to obtain a further thinwide band gap semiconductor wafer and to obtain a remaining wide bandgap semiconductor boule.

An example relates to a wide band gap semiconductor wafer. The wide bandgap semiconductor wafer comprises a monocrystalline wide band gapsemiconductor layer. A thickness of the monocrystalline wide band gapsemiconductor layer is at least 250 μm. The wide band gap semiconductorwafer further comprises a non-monocrystalline support layer located at asurface of the monocrystalline semiconductor substrate. A thickness ofthe non-monocrystalline support layer is at least 150 μm. A thermalexpansion coefficient of the non-monocrystalline support layer differsfrom a thermal expansion coefficient of the monocrystalline wide bandgap semiconductor layer by at most 5% of the thermal expansioncoefficient of the monocrystalline wide band gap semiconductor layer.

BRIEF DESCRIPTION OF THE FIGURES

Some examples of apparatuses and/or methods will be described in thefollowing by way of example only, and with reference to the accompanyingfigures, in which

FIG. 1 shows a flow chart of a method for processing a wide band gapsemiconductor wafer;

FIG. 2 shows a flow chart of a method for forming a plurality of thinwide band gap semiconductor wafers;

FIG. 3 shows schematic cross section of a wide band gap semiconductorwafer with a non-monocrystalline support layer; and

FIGS. 4a to 4i show an example of a method for processing a wide bandgap semiconductor wafer.

DETAILED DESCRIPTION

Various examples will now be described more fully with reference to theaccompanying drawings in which some examples are illustrated. In thefigures, the thicknesses of lines, layers and/or regions may beexaggerated for clarity.

Accordingly, while further examples are capable of various modificationsand alternative forms, some particular examples thereof are shown in thefigures and will subsequently be described in detail. However, thisdetailed description does not limit further examples to the particularforms described. Further examples may cover all modifications,equivalents, and alternatives falling within the scope of thedisclosure. Same or like numbers refer to like or similar elementsthroughout the description of the figures, which may be implementedidentically or in modified form when compared to one another whileproviding for the same or a similar functionality.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, the elements may bedirectly connected or coupled or via one or more intervening elements.If two elements A and B are combined using an “or”, this is to beunderstood to disclose all possible combinations, i.e. only A, only B aswell as A and B, if not explicitly or implicitly defined otherwise. Analternative wording for the same combinations is “at least one of A andB” or “A and/or B”. The same applies, mutatis mutandis, for combinationsof more than two Elements.

The terminology used herein for the purpose of describing particularexamples is not intended to be limiting for further examples. Whenever asingular form such as “a,” “an” and “the” is used and using only asingle element is neither explicitly or implicitly defined as beingmandatory, further examples may also use plural elements to implementthe same functionality. Likewise, when a functionality is subsequentlydescribed as being implemented using multiple elements, further examplesmay implement the same functionality using a single element orprocessing entity. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when used,specify the presence of the stated features, integers, steps,operations, processes, acts, elements and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, processes, acts, elements, componentsand/or any group thereof.

Unless otherwise defined, all terms (including technical and scientificterms) are used herein in their ordinary meaning of the art to which theexamples belong.

Hereinafter, a “semiconductor boule” may be a semiconductor ingot and/ora thick semiconductor wafer. For example, a semiconductor boule may havethe shape of an elongated rod or an elongated bar. A thick semiconductorwafer may have the shape of a disc or a cylinder. A thickness of a thicksemiconductor wafer may be at least 2 mm. Furthermore, a “semiconductorwafer” may be a disc of a semiconductor material. A thickness of saiddisc may be at least one order of magnitude, for example at least twoorders of magnitude, smaller than an extension of said discperpendicular to a thickness. In the case of a thick semiconductorwafer, the thickness may be approximately one order of magnitude smallerthan the extension of the thick semiconductor wafer perpendicular to thethickness.

FIG. 1 shows a flow chart of an embodiment of a method 100 forprocessing a wide band gap semiconductor wafer. The method 100 maycomprise depositing 110 a non-monocrystalline support layer on the wideband gap semiconductor wafer. For example, the non-monocrystallinesupport layer is deposited 110 at a back side of the wide band gapsemiconductor wafer. The non-monocrystalline support layer may be acrystalline layer, e.g. a polycrystalline layer or an amorphous layer.The non-monocrystalline support layer may comprise semiconductormaterial, e.g. a wide band gap semiconductor. For example, thenon-monocrystalline support layer may be deposited directly on a backside surface of the wide band gap semiconductor wafer. Alternatively, anintermediate layer may be formed between the non-monocrystalline supportlayer and the back side surface of the wide band gap semiconductorwafer.

For example, the method 100 may comprise depositing 120 an epitaxiallayer at a front side of the wide band gap semiconductor wafer oppositeto the back side of the wide band gap semiconductor wafer. The epitaxiallayer may be epitaxially grown directly on a front side surface of thewide band gap semiconductor wafer, for example. For example, only thewide band gap semiconductor wafer may be located between thenon-monocrystalline support layer and the epitaxial layer. For example,the epitaxial layer may be grown on a same wide band gap semiconductormaterial as the non-monocrystalline support layer. For example, theepitaxial layer may be grown directly on a monocrystalline portion ofthe wide band gap semiconductor wafer, for example.

The method 100 may further comprise splitting 130 the wide band gapsemiconductor wafer along a splitting region. Splitting 130 may beperformed to obtain a device wafer comprising at least a part of theepitaxial layer. For example, the splitting region may be located withinthe epitaxial layer so that only a part of the epitaxial layer remainsfor the device wafer. Alternatively, the splitting region may be locatedwithin the wide band gap semiconductor wafer so that the device wafermay comprise the complete epitaxial layer.

The splitting region may be located within a monocrystalline portion(e.g. a monocrystalline layer) of the wide band gap semiconductor wafer,for example. The device wafer may comprise monocrystalline wide band gapsemiconductor material (e.g. the device wafer may be a monocrystallinesemiconductor wafer). The device wafer may be a monocrystalline wideband gap semiconductor wafer, e.g. the device wafer may comprise lessthan 10% (or less than 5%, less than 3%, less than 1% or less than 0.5%)of non-monocrystalline semiconductor material, for example aftersplitting 130 the device wafer. For example, the device wafer maycomprise the (e.g. monocrystalline) epitaxial layer and a (e.g.monocrystalline) portion of the split wide band gap semiconductor wafer.

Further, a remaining wafer may be obtained by splitting 130 the wideband gap semiconductor wafer. The remaining wafer may comprise thenon-monocrystalline support layer. Further, the remaining layer maycomprise a remaining portion of the wide band gap semiconductor wafer,e.g. a monocrystalline layer of the wide band gap semiconductor wafer.Splitting 130 may be performed after depositing 110 thenon-monocrystalline support layer, for example.

For example, the device wafer may have a thickness of more than 2 times(or more than 3 times, more than 5 times or more than 10 times) athickness of the remaining wafer. For example, a thickness of theremaining wide band gap semiconductor wafer of the remaining wafer maybe at least 2 times a thickness of the split device wafer.

For example, providing the non-monocrystalline support layer may enablefurther splitting the remaining wafer or the monocrystalline wide bandgap semiconductor wafer substrate of the remaining wafer. Thenon-monocrystalline support layer may provide mechanical support for thewide band gap semiconductor wafer and/or may increase the totalthickness of the wide band gap semiconductor wafer to enable handling ofthe wide band gap semiconductor wafer. The non-monocrystalline supportlayer may enable further usage of the wide band gap semiconductor wafersubstrate, e.g. the complete wide band gap semiconductor wafer substratemay be used. For example, in further splitting processes further devicewafers may be formed. Using the complete wide band gap semiconductorwafer substrate may reduce costs, since the costs for a new devicewafer, e.g. thin wide band gap semiconductor wafers, may be higher thanthe costs for forming the non-monocrystalline support layer. Forexample, the non-monocrystalline support layer may be easier and/orfaster and/or cheaper to form than a monocrystalline wide band gapsemiconductor wafer.

Splitting the wide band gap semiconductor wafer may be more complex thanthinning a wafer by grinding, for example. However, splitting may enableto further use a remaining portion of the wide band gap semiconductorwafer, for example. Grinding may be used for thinning other materials,e.g. non-monocrystalline wafers or non-monocrystalline layers (e.g.amorphous layers), e.g. for economic reasons.

For example, a thermal expansion coefficient of the non-monocrystallinesupport layer may differ from a thermal expansion coefficient of thewide band gap semiconductor wafer by at most 10% (or by at most 5%, byat most 3%, by at most 1% or by at most 0.1%) of the thermal expansioncoefficient of the wide band gap semiconductor wafer. By providing thenon-monocrystalline support layer with a thermal expansion coefficientsimilar or equal to the thermal expansion coefficient of the wide bandgap semiconductor wafer, deforming of the wide band gap semiconductorwafer supported by the non-monocrystalline support layer may be avoided.For example, the similar thermal expansion coefficients may enableprocessing the wide band gap semiconductor wafer supported by thenon-monocrystalline support layer at different temperatures.

For example, the non-monocrystalline support layer may be deposited witha deposition rate of at least 40 μm/hour (or at least 50 μm/hour, atleast 60 μm/hour, at least 80 μm/hour or at least 100 μm/hour). Forexample, chemical vapor deposition (CVD), laser CVD and/or Close SpaceEpitaxy may be used for depositing the non-monocrystalline supportlayer. For example, a high deposition rate may be usable as only amechanical stability of the non-monocrystalline support layer may berequired, but there might be no electrical requirements to thenon-monocrystalline support layer. A high deposition rate may enablefast processing and thus may reduce manufacturing costs.

For example, a protective layer may be located at the front side of thewide band gap semiconductor wafer during depositing thenon-monocrystalline support layer. The protective layer (e.g. a carboncap) may be formed at the front side or deposited on the front sidebefore depositing the non-monocrystalline support layer. The protectivelayer may be removed after depositing the non-monocrystalline supportlayer, for example to enable depositing the epitaxial layer at the frontside. The protective layer may prevent growth of a layer at the frontside of the wide band gap semiconductor wafer while depositing thenon-monocrystalline support layer, for example.

For example, the non-monocrystalline support layer may be one of apoly-silicon carbide layer and a molybdenum layer, if the wide band gapsemiconductor wafer is a silicon carbide wafer. For example, thenon-monocrystalline support layer may comprise at least two sublayers ofdifferent materials. For example, the non-monocrystalline support layermay comprise a first sublayer of poly-silicon carbide and a secondsublayer of molybdenum and/or carbon, for example graphite.

For example, a total thickness of the remaining wafer including thenon-monocrystalline support layer (and e.g. a remaining portion of thewide band gap semiconductor wafer) may be at least 200 μm (or at least300 μm, at least 400 μm, or at least 500 μm) and/or at most 1500 μm (orat most 1300 μm, at most 1000 μm, or at most 700 μm). For example, thenon-monocrystalline support layer may be deposited with a predefinedthickness so that a sufficient total thickness of the remaining wafermay be achieved after splitting the wide band gap semiconductor wafer.For example, the total thickness of the remaining wafer may be adaptedby the deposition of the non-monocrystalline support layer to enablefurther processing the remaining wafer with standard semiconductorequipment.

For example, the method may comprise depositing a furthernon-monocrystalline support layer on the non-monocrystalline supportlayer of the remaining wafer, e.g. directly on the non-monocrystallinesupport layer of the remaining wafer. For example, after depositing thenon-monocrystalline support layer, the wide band gap semiconductor wafer(or a respective remaining wafer) may be split several times to obtainseveral device wafers. The further non-monocrystalline support layer maybe deposited, for example, if the thickness of a respective remainingwafer would be lower than the minimum total thickness of the wide bandgap semiconductor wafer which is required for the processing of thewafer, e.g. after a splitting a further device wafer. Depositing thefurther non-monocrystalline support layer may enable further processingthe respective remaining wafer, e.g. after splitting the further devicewafer.

For example, a material of the further non-monocrystalline support layermay differ from a material of the non-monocrystalline support layer. Forexample, using different materials may facilitate depositing therespective non-monocrystalline support layer and/or may reduce costs ofdepositing the non-monocrystalline support layer.

For example, a thickness of the remaining wafer including thenon-monocrystalline support layer and the further non-monocrystallinesupport layer may differ by at most 400 μm (or by at most 300 μm, or byat most 200 μm) from a thickness of the remaining wide band gapsemiconductor wafer before splitting the wide band gap semiconductorwafer, e.g. before or after depositing the epitaxial layer. For example,the thickness of the further non-monocrystalline support layer may besimilar to a thickness of the device wafer so that an overall thicknessof the wide band gap semiconductor wafer or the respective remainingwafers may remain approximately constant while splitting device wafersfrom the wide band gap semiconductor wafer or the respective remainingwafers. For example, the thickness of the further non-monocrystallinesupport layer may depend on a thickness of the device wafer and/or athickness of the epitaxial layer so that an overall thickness of thewide band gap semiconductor wafer may remain constant while splittingdevice wafers from the wide band gap semiconductor wafer. For example,after depositing a non-monocrystalline support layer, two or more devicewafers may be split from the wide band gap semiconductor wafer and thefurther non-monocrystalline support layer may have a thickness dependingon a total thickness of the two or more device wafers.

For example, the method may comprise depositing a further epitaxiallayer at a front side of the remaining wafer. The method may furthercomprise splitting the remaining wafer along a further splitting regionto obtain a further device wafer comprising the further epitaxial layer,and to obtain a further remaining wafer comprising thenon-monocrystalline support layer. The method may comprise depositing afurther non-monocrystalline support layer on the non-monocrystallinesupport layer of the further remaining wafer, for example after thefurther splitting of the remaining wafer. The furthernon-monocrystalline support layer may have a thickness of at least 70%(or at least 80% or at least 90%) and/or of at most 130% (or of at most120% or of at most 110%) of the total thickness of the further devicewafer. The thickness of the device wafer may be at most 300 μm (or atmost 200 μm, at most 110 μm, at most 50 μm or at most 20 μm), forexample. For example, the further non-monocrystalline support layer maybe deposited before splitting the remaining wafer to obtain the furtherdevice wafer.

According to an example, the method may further comprise forming adoping region of a wide band gap semiconductor device in the wide bandgap semiconductor wafer, e.g. in the epitaxial layer. The doping regionmay be formed after depositing the non-monocrystalline support layer.The doping region may be an anode region or cathode region of a diode ora source region, a body region, a drain region, an emitter region, abase region or a collector region of a transistor (e.g. MOSFET or IGBT).The doping region may be an n-doped or p-doped region. For example, thedoping region may be formed before splitting the wide band gapsemiconductor wafer. For example, a plurality of doping regions of acorresponding plurality of wide band gap semiconductor devices to beformed on the wide band gap semiconductor wafer may be formedsimultaneously.

For example, the method may further comprise forming a metallizationstructure of the wide band gap semiconductor device at the front side ofthe wide band gap semiconductor wafer, e.g. after depositing thenon-monocrystalline support layer. For example, respective metallizationstructures for the plurality of wide band gap semiconductor devices maybe formed. For example, the metallization structure may be formed beforesplitting the wide band gap semiconductor wafer.

For example, semiconductor processes may be performed at temperatures ofmore than 1000° C. before splitting. For example, the device wafer isprocessed at temperatures of at most 1000° C. after splitting to avoidaltering of structures formed at the front side before splitting.

For example, one or more wide band gap semiconductor devices may beformed on the wide band gap semiconductor wafer. For example, each wideband gap semiconductor device comprises a transistor. At least one of agate trench, and a gate electrode of the transistor may be formed beforesplitting.

The transistor may be a field effect transistor (e.g. a metal oxidesemiconductor field effect transistor (MOSFET) or an insulated gatebipolar transistor (IGBT)). The gate of the transistor may be located ina gate trench extending into the wide band gap semiconductor substrateor may be located on a lateral surface of the wide band gapsemiconductor substrate. The transistor may comprise one or moretransistor cells. For example, the wide band gap semiconductor substratemay comprise one or more source regions, one or more body regions and adrift region of the transistor. The one or more source regions and thedrift region may each be of a first conductivity type (e.g. n-doped).The one or more body regions may be of a second conductivity type (e.g.p-doped).

The transistor may be a vertical transistor structure conducting currentbetween a front side surface of the wide band gap semiconductorsubstrate and a back side surface of the wide band gap semiconductorsubstrate. For example, the transistor of the wide band gapsemiconductor device may comprise a plurality of source doping regionsconnected to a source wiring structure, a plurality of gate electrodesor a gate electrode grid connected to a gate wiring structure and a backside drain metallization or back side collector metallization.

For example, the wide band gap semiconductor wafer may be either one of:a wide band gap semiconductor base substrate, a wide band gapsemiconductor base substrate with a wide band gap semiconductorepitaxial layer grown on the wide band gap semiconductor base substrateor a wide band gap semiconductor epitaxial layer. The wide band gapsemiconductor wafer may be a monocrystalline wafer or may comprise atleast a monocrystalline wide band gap semiconductor layer.

For example, the wide band gap semiconductor wafer may have a band gaplarger than the band gap of silicon (1.1 eV). In particular, the wideband gap semiconductor wafer has a band gap larger than 2 eV, forexample larger than 3 eV. For example, the wide band gap semiconductorwafer may be a silicon carbide semiconductor (SiC) wafer, or galliumarsenide (GaAs) semiconductor wafer, or a gallium nitride (GaN)semiconductor wafer.

The front side of the wide band gap semiconductor wafer may be the sideused to implement more sophisticated and complex structures (e.g. gatesof transistors) than at the back side of the semiconductor substrate.The process parameters (e.g. temperature) and the handling may belimited for forming structures at the back side to avoid altering ofstructures formed at the front side.

A wide band gap semiconductor device to be formed on the wide band gapsemiconductor wafer may be a power semiconductor device. A powersemiconductor device or an electrical structure (e.g. transistorarrangement of the semiconductor device) of the power semiconductordevice may have a breakdown voltage or blocking voltage of more than 100V (e.g. a breakdown voltage of 200 V, 300 V, 400V or 500V) or more than500 V (e.g. a breakdown voltage of 600 V, 700 V, 800V or 1000V) or morethan 1000 V (e.g. a breakdown voltage of 1200 V, 1500 V, 1700V, 2000V,3300V or 6500V), for example.

FIG. 2 shows a flow chart of a method 200 for forming a plurality ofthin wide band gap semiconductor wafers. The method 200 may comprisedepositing 210 a first non-monocrystalline support layer on a wide bandgap semiconductor boule. The non-monocrystalline support layer may be apolycrystalline layer and/or an amorphous layer, for example.

Further, the method 200 may comprise separating 220 (e.g. splitting 220and/or sawing 220) the wide band gap semiconductor boule along a firstseparating region (e.g. splitting region or sawing region) to obtain afirst thin wide band gap semiconductor wafer. Alternatively oradditionally to splitting 220, the first thin wide band gapsemiconductor wafer may be obtained by sawing 220 the wide band gapsemiconductor boule, e.g. partly. The thin wide band gap semiconductorwafer may comprise the non-monocrystalline support layer and a thin wideband gap semiconductor layer. A thickness of the non-monocrystallinesupport layer may be at least 100 μm (or at least 150 μm), for example.The thin wide band gap semiconductor layer may have a thickness of atleast 50 μm (of at least 100 μm or of at least 250 μm) and/or at most500 μm, for example. Further, a first remaining wide band gapsemiconductor boule may be obtained. For example, a thickness of thewide band gap semiconductor boule may be at least 2 times (or at least 5times, or at least 10 times) a thickness of the thin wide band gapsemiconductor layer.

Further, the method 200 may comprise depositing 230 a further (e.g. asecond) non-monocrystalline support layer on the first remaining wideband gap semiconductor boule, e.g. comprising the non-monocrystallinesupport layer. The further non-monocrystalline support layer may bedeposited 230 after separating 220 (e.g. splitting or sawing) the wideband gap semiconductor boule along the first separating region, forexample.

The method 200 may further comprise separating 240 (e.g. splitting 240or sawing 240) the first remaining wide band gap semiconductor boulealong a second separating region (e.g. splitting region or sawingregion). For example, a further (e.g. a second) thin wide band gapsemiconductor wafer and a further (e.g. a second) remaining wide bandgap semiconductor boule may be obtained. The further thin wide band gapsemiconductor wafer may comprise the further non-monocrystalline supportlayer and a further thin wide band gap semiconductor layer, for example.This process sequence may be repeated several times.

For example, a further (e.g. a third) non-monocrystalline support layermay be deposited on the second remaining wide band gap semiconductorboule. The second remaining wide band gap semiconductor boule may beseparated (e.g. split or sawed) along a further separating region (e.g.splitting region or sawing region) to obtain a further (e.g. a third)thin wide band gap semiconductor wafer, and a further remaining wideband gap semiconductor boule, for example. The further thin wide bandgap semiconductor wafer may comprise the further non-monocrystallinesupport layer and a thin wide band gap semiconductor layer.Consequently, further non-monocrystalline support layers may bedeposited on respective further remaining wide band gap semiconductorboules to form a plurality of thin wide band gap semiconductor wafers.The process may be repeated, e.g. several times, e.g. until a thicknessof a remaining wide band gap semiconductor boule is too small to splitthe remaining wide band gap semiconductor boule, for example if it isthinner than the thin wide band gap semiconductor wafer.

The method 200 may be used to provide or split or saw thin wide band gapsemiconductor wafers from a wide band gap semiconductor boule. Thereby,the amount of wide band gap semiconductor material required for a thinwide band gap semiconductor wafer may be reduced. It may be possiblethat mechanical stability (e.g. for further processing) is provided bythe non-monocrystalline support layer. The wide band gap semiconductorboule may have a thickness of at least 2 mm (or of at least 3 mm or ofat least 5 mm) for example. The wide band gap semiconductor boule may bea wide band gap semiconductor ingot and/or a thick wide band gapsemiconductor wafer. For example, the wide band gap semiconductor boulemay have the shape of an elongated rod or an elongated bar. The method200 may enable providing thin wide band gap semiconductor wafers whiledecreasing a needed amount of wide band gap semiconductor wafermaterial, for example.

More details and aspects are mentioned in connection with theembodiments described above or below. The embodiments shown in FIG. 2may comprise one or more optional additional features corresponding toone or more aspects mentioned in connection with the proposed concept orone or more embodiments described above or below (e.g. FIGS. 1 and 3-4i).

FIG. 3 shows an illustration of a wide band gap semiconductor wafer 300.The wide band gap semiconductor wafer 300 may comprise a monocrystallinewide band gap semiconductor layer 310. A thickness of themonocrystalline wide band gap semiconductor layer may be at least 50 μm(or at least 70 μm, at least 90 μm, at least 100 μm, at least 150 μm, atleast 250 μm, at least 350 μm, at least 500 μm, at least 700 μm or atleast 1000 μm) and/or at most 2000 μm (or at most 1300 μm, at most 1000μm or at most 800 μm). For example, the monocrystalline wide band gapsemiconductor layer having a thickness of more than 300 μm may be usedfor splitting a plurality of thin monocrystalline semiconductor wafersfrom the monocrystalline wide band gap semiconductor layer 310.

The wide band gap semiconductor wafer 300 may further comprise anon-monocrystalline support layer. The non-monocrystalline support layermay be a crystalline layer, e.g. an amorphous layer or apoly-crystalline layer. For example, the non-monocrystalline supportlayer may comprise a semiconductor material, e.g. a wide band gapsemiconductor. The non-monocrystalline support layer may be located at asurface of the monocrystalline semiconductor layer. A thickness of thenon-monocrystalline support layer may be at least 100 μm (or at least150 μm, at least 200 μm, at least 250 μm, at least 300 μm, at least 400μm, at least 500 μm, at least 700 μm or at least 1000 μm) and/or at most1500 μm (or at most 1200 μm, at most 1000 μm, at most 750 μm at most 500μm, at most 350 μm or at most 200 μm). A thickness of thenon-monocrystalline support layer of more than 300 μm (e.g. more than350 μm) may for example enable splitting further thin semiconductorwafers from the monocrystalline wide band gap semiconductor layer 310.

For example, the non-monocrystalline support layer increases a totalthickness of the wide band gap semiconductor wafer 300 and maymechanically support thin monocrystalline wide band gap semiconductorlayers, e.g. with a thickness of the monocrystalline wide band gapsemiconductor layer smaller than 250 μm. For example, the totalthickness of the wide band gap semiconductor wafer 300 is at least 200μm (or at least 250 μm) and/or at most 1500 μm (or at most 1300 μm). Thetotal thickness of the wide band gap semiconductor wafer 300 may enableprocessing the wide band gap semiconductor wafer 300 using standardsemiconductor equipment.

Providing the non-monocrystalline support layer may enable processingand/or using the complete monocrystalline wide band gap semiconductorlayer. The monocrystalline wide band gap semiconductor layer maycomprise an expensive material whereas the material of thenon-monocrystalline support layer may be less expensive. For example,the wide band gap semiconductor wafer 300 may be a silicon carbide waferand the monocrystalline wide band gap semiconductor layer may be asilicon carbide layer. For example, the non-monocrystalline supportlayer may be a poly silicon carbide layer. By providing thenon-monocrystalline support layer, the monocrystalline wide band gapsemiconductor layer may be completely used or processed, e.g. bysplitting thin wide band gap semiconductor wafers from themonocrystalline wide band gap semiconductor layer. For this, additionalpoly silicon carbide layers may be deposited on the supporting layer assoon as the critical thickness for the further processing of the waferis reached. A non-usable remaining part of the monocrystalline wide bandgap semiconductor layer may be reduced (for example to a thickness ofless than 60 μm or of less than 30 μm) so that by using themonocrystalline wide band gap semiconductor layer more efficiently,overall manufacturing costs of thin wide band gap semiconductor wafersmay be reduced, for example.

For example, a thermal expansion coefficient of at least a part of thenon-monocrystalline support layer may differ from a thermal expansioncoefficient of the monocrystalline wide band gap semiconductor layer byat most 10% (or by at most 5%, by at most 3%, by at most 1% or by atmost 0.1%) of the thermal expansion coefficient of the monocrystallinewide band gap semiconductor layer. The part of the non-monocrystallinesupport layer may be a sublayer of the non-monocrystalline supportlayer, for example. The similar thermal expansion coefficient of the twolayers may result in preventing wafer bow of the wide band gapsemiconductor wafer 300, e.g. when processing the wide band gapsemiconductor wafer 300 at different temperatures.

For example, a first sublayer of the non-monocrystalline support layermay comprise a first material and a second sublayer of thenon-monocrystalline support layer may comprise a second material,wherein the first material may differ from the second material. Forexample, the first and second material may have different thermalexpansion coefficients. For example, the thermal expansion coefficientof the sublayer next to the monocrystalline wide band gap semiconductorlayer may differ from the thermal expansion coefficient of themonocrystalline wide band gap semiconductor layer by at most 10% of thethermal expansion coefficient of the monocrystalline wide band gapsemiconductor layer, and a thermal expansion coefficient of anothersublayer may differ from the thermal expansion coefficient of themonocrystalline wide band gap semiconductor layer by more than 10%, forexample.

More details and aspects are mentioned in connection with theembodiments described above or below. The embodiments shown in FIG. 3may comprise one or more optional additional features corresponding toone or more aspects mentioned in connection with the proposed concept orone or more embodiments described above or below (e.g. FIGS. 1-2 and 4a-4 i).

FIGS. 4a to 4i show an example of a method for processing a wide bandgap semiconductor wafer 400. The wide band gap semiconductor wafer 400may be provided as start wafer. An epitaxial layer 410 may be depositedon the start wafer. For example, front side structures or metallizationstructures 420, 422 may be formed on the epitaxial layer 410. Forexample, after forming the front side structures, e.g. to providesemiconductor devices, the wide band gap semiconductor wafer 400 may besplit. For example, as shown in FIG. 4d , a remaining bulk wafer 430 maybe obtained after splitting, the remaining bulk wafer 430 comprising atleast a first part of the start wafer. For example, a device wafer 440may be obtained after splitting. The device wafer 440 may comprise theepitaxial layer 410 and the metallization structures 420, 422. Forexample, the device wafer 440 may further comprise a second part of theof the start wafer. The second part of the start wafer may be thinnerthan the epitaxial layer 410, for example.

The bulk wafer 430 may be turned around so that a front side 450 of thebulk wafer 430 faces downwards. The bulk wafer 430 may be thickened by asupport layer 460 formed by deposition at a back side of the bulk wafer430 opposite to the front side 450. The bulk wafer 430 and the supportlayer 460 may form a new start wafer 470. For example, the new startwafer 470 may be turned around so that the front side 450 faces upwards,e.g. to enable further processing of the front side. A process for areduction of the surface roughness may be performed on the first side450, like e.g. CMP, etching, polishing. A further epitaxial layer may beformed at the front side 450, e.g. after a post-processing of the newstart wafer 470, and an iteration of the method may be performed,wherein the new start wafer 470 may substitute the start wafer 400.

More details and aspects are mentioned in connection with theembodiments described above or below. The embodiments shown in FIGS. 4ato 4i may comprise one or more optional additional featurescorresponding to one or more aspects mentioned in connection with theproposed concept or one or more embodiments described above or below(e.g. FIGS. 1-3).

For example, a limitation regarding reuse cycles may occur for someconcepts concerning splitting and reuse, and it might not be possible tocompletely process a substrate of the thin semiconductor wafer. Forexample, a remaining wafer part may have a thickness lower than 250 μmafter splitting the semiconductor wafer. Due to its low thickness itmight not be possible to further use the remaining part. To guarantee arequired thickness of the remaining part, e.g. larger than 250 μm, athick epitaxial layer may be grown on the semiconductor wafer beforesplitting, the epitaxial layer resulting in increased costs. There maybe a demand for improved concepts for processing semiconductor wafers.

According to other concepts, it might not be possible to completelyprocess a wafer below a thickness of 250 μm. For concepts which aim atsplitting and/or reuse of the wafer, thus a limitation regarding re-usecycles may result. E.g., a start substrate (e.g. the start wafer) has astart thickness of 350 μm, and a 110 μm split thickness may be used.According to some concepts, a maximum of 2× splitting (e.g. incl.>30 μmEpi (Epi: epitaxial) deposition) of the start substrate may be achieved,even if a rest of the bulk could still be used.

For example, by using the provided concepts, also the remaining part ofthe bulk may be completely used. A proposed method e.g. may provide theuse of a temperature-resistant and/or low-cost deposition on the waferback side. This deposition e.g. may only serve for thickening the disc,e.g. the wafer or the substrate, and might have no electric relevance,as this layer is e.g. not found again in the resulting device wafer(which is split off, for example). Still, this additional layer mightnot negatively influence the manufacturability of the wafers duringprocessing, both regarding wafer bow/warp and roughness.

For example, the deposition of poly silicon carbide (poly-SiC) on thewafer back side may take place from the gas phase, e.g. similar gassystems may be used as in SiC epitaxy (e.g. hydrocarbons, silanes and/orsilane derivatives and/or gases containing carbon and silicon (Si) inthe correct ratio). Also the use of chlorine (Cl)—containing gases maybe sensible (e.g. trichlorosilane), e.g. in order to acquire highsurface diffusion rates and consequently high growth rates. Furtheralternatives may be e.g. the use of laser chemical vapor deposition(CVD) or Close Space Epitaxy.

For example, the monocrystalline wafer front side may be protected by aremovable cover layer (e.g. Carbon Cap), e.g. to suppress an undesirablegrowth there. A possible implementation would be, e.g. by adapting theused processing equipment, to select the overall thickness of the startwafer to be clearly thicker than 350 micrometers and possibly minimizethe wafer bow and facilitate wafer handling. In this respect, at thestart of processing the initial wafer the same may be provided with apoly silicon carbide layer of a corresponding thickness which may beincreased during the later re-use processes so that the wafer may bere-used several times, for example. This process may be repeated untilthe original mono-crystalline SiC layer is used up. Thickening might nothave to be executed after every splitting process, e.g. in case of polysilicon carbide, but e.g. only when used processing equipment makes itnecessary.

An example relates to a method for thickening a SiC wafer. An aspect mayrelate to the wafer back-side deposition of a layer compatible with SiCprocesses to consequently prevent falling short of the critical waferthickness with SiC wafer re-use concepts by means of splitting offlayers, for example. This split-off layer may not only fulfil thermalbut also mechanical requirements. An example for such a layer may be apoly SiC layer.

Using the proposed concepts may enable to reduce manufacturing costs forSiC technologies as a substrate may be re-used several times. Proposedconcepts may also be integrated for even thinner split layers and/orscalable to different wafer diameters.

Thick epitaxial layer depositions on the wafer front side may enable athickening of the discs/layers. Compared to other concepts, proposedconcepts may be easily implemented and/or may be easy to include intoproduction process and/or may enable the implementation of currentlyrequired testing concepts.

The aspects and features mentioned and described together with one ormore of the previously detailed examples and figures, may as well becombined with one or more of the other examples in order to replace alike feature of the other example or in order to additionally introducethe feature to the other example.

The description and drawings merely illustrate the principles of thedisclosure. Furthermore, all examples recited herein are principallyintended expressly to be only for illustrative purposes to aid thereader in understanding the principles of the disclosure and theconcepts contributed by the inventor(s) to furthering the art. Allstatements herein reciting principles, aspects, and examples of thedisclosure, as well as specific examples thereof, are intended toencompass equivalents thereof.

It is to be understood that the disclosure of multiple acts, processes,operations, steps or functions disclosed in the specification or claimsmay not be construed as to be within the specific order, unlessexplicitly or implicitly stated otherwise, for instance for technicalreasons. Therefore, the disclosure of multiple acts or functions willnot limit these to a particular order unless such acts or functions arenot interchangeable for technical reasons. Furthermore, in some examplesa single act, function, process, operation or step may include or may bebroken into multiple sub-acts, -functions, -processes, -operations or-steps, respectively. Such sub acts may be included and part of thedisclosure of this single act unless explicitly excluded.

Furthermore, the following claims are hereby incorporated into thedetailed description, where each claim may stand on its own as aseparate example. While each claim may stand on its own as a separateexample, it is to be noted that—although a dependent claim may refer inthe claims to a specific combination with one or more other claims—otherexamples may also include a combination of the dependent claim with thesubject matter of each other dependent or independent claim. Suchcombinations are explicitly proposed herein unless it is stated that aspecific combination is not intended. Furthermore, it is intended toinclude also features of a claim to any other independent claim even ifthis claim is not directly made dependent to the independent claim.

What is claimed is:
 1. A method for processing a wide band gapsemiconductor wafer, the method comprising: depositing anon-monocrystalline support layer comprising semiconductor material at aback side of a wide band gap semiconductor wafer, the wide band gapsemiconductor wafer having a band gap larger than the band gap ofsilicon; depositing an epitaxial layer at a front side of the wide bandgap semiconductor wafer; and splitting the wide band gap semiconductorwafer along a splitting region to obtain a device wafer comprising atleast a part of the epitaxial layer, and a remaining wafer comprisingthe non-monocrystalline support layer.
 2. The method of claim 1, whereina thermal expansion coefficient of the non-monocrystalline support layerdiffers from a thermal expansion coefficient of the wide band gapsemiconductor wafer by at most 10% of the thermal expansion coefficientof the wide band gap semiconductor wafer.
 3. The method of claim 1,wherein the non-monocrystalline support layer is deposited at adeposition rate of at least 50 μm/hour.
 4. The method of claim 1,wherein the non-monocrystalline support layer is a poly-silicon carbidelayer or a molybdenum layer.
 5. The method of claim 1, wherein a totalthickness of the remaining wafer including the non-monocrystallinesupport layer is at least 200 μm and at most 1500 μm.
 6. The method ofclaim 1, wherein a protective layer is located at the front side of thewide band gap semiconductor wafer during the depositing of thenon-monocrystalline support layer.
 7. The method of claim 1, furthercomprising: depositing a further non-monocrystalline support layer onthe non-monocrystalline support layer of the remaining wafer.
 8. Themethod of claim 7, wherein a material of the further non-monocrystallinesupport layer differs from a material of the non-monocrystalline supportlayer.
 9. The method of claim 7, wherein a thickness of the remainingwafer including the non-monocrystalline support layer and the furthernon-monocrystalline support layer differs by at most 300 μm from athickness of the wide band gap semiconductor wafer before the splittingof the wide band gap semiconductor wafer.
 10. The method of claim 7,wherein the further non-monocrystalline support layer has a thickness ofat least 90% and of at most 110% of the total thickness of the furtherdevice wafer.
 11. The method of claim 1, further comprising: depositinga further epitaxial layer at a front side of the remaining wafer; andsplitting the remaining wafer along a further splitting region to obtaina further device wafer comprising the further epitaxial layer and afurther remaining wafer comprising the non-monocrystalline supportlayer.
 12. The method of claim 1, further comprising: forming a dopingregion of a wide band gap semiconductor device in the wide band gapsemiconductor wafer after the depositing of the non-monocrystallinesupport layer.
 13. The method of claim 1, further comprising: forming ametallization structure of the wide band gap semiconductor device at thefront side of the wide band gap semiconductor wafer after the depositingof the non-monocrystalline support layer.
 14. The method of claim 1,wherein at least one of a gate trench and a gate electrode of atransistor is formed at the front side of the wide band gapsemiconductor wafer before the splitting.
 15. The method of claim 1,wherein the device wafer is processed at temperatures of at most 1000°C. after the splitting.
 16. The method of claim 1, wherein the wide bandgap semiconductor wafer is a silicon carbide wafer.
 17. A method forforming a plurality of thin wide band gap semiconductor wafers, themethod comprising: depositing a first non-monocrystalline support layercomprising semiconductor material on a wide band gap semiconductorboule, the wide band gap semiconductor boule having a band gap largerthan the band gap of silicon; separating the wide band gap semiconductorboule along a first separating region to obtain a first thin wide bandgap semiconductor wafer and a first remaining wide band gapsemiconductor boule, the first thin wide band gap semiconductor wafercomprising the first non-monocrystalline support layer and a thin wideband gap semiconductor layer, a thickness of the wide band gapsemiconductor boule being at least 2 times a thickness of the thin wideband gap semiconductor layer; depositing a second non-monocrystallinesupport layer on the first remaining wide band gap semiconductor boule;and separating the first remaining wide band gap semiconductor boulealong a second separating region to obtain a second thin wide band gapsemiconductor wafer and a second remaining wide band gap semiconductorboule, the second thin wide band gap semiconductor wafer comprising thesecond non-monocrystalline support layer and a thin wide band gapsemiconductor layer.
 18. The method of claim 17, further comprising:depositing a third non-monocrystalline support layer on the secondremaining wide band gap semiconductor boule; and separating the secondremaining wide band gap semiconductor boule along a third separatingregion to obtain a third thin wide band gap semiconductor wafer and athird remaining wide band gap semiconductor boule, the third thin wideband gap semiconductor wafer comprising the third non-monocrystallinesupport layer and a thin wide band gap semiconductor layer.
 19. Themethod of claim 17, wherein the separating comprises at least one ofsplitting and sawing.